Percent Purity Problems: Finding Mass of Product from Impure Samples
Chemistry · Some Basic Concepts Of Chemistry · NEET
In a purity problem, only the pure part of a sample reacts; the impurity does nothing. So first find the mass of pure substance = (total mass) x (% purity / 100), then do normal stoichiometry with that pure mass. Memory hook: "Impurity is a passenger, not a driver" - only the pure part drives the reaction.
Two-step method for impure sample problems: convert the impure mass to pure mass using purity, then do normal stoichiometry. The impurity (yellow) is inert and does not react.
Your doubts, answered
What does "95% pure CaCO3" actually mean?
It means that in every 100 g of the sample, only 95 g is real CaCO3. The other 5 g is impurity (like sand or dirt) that does NOT react. So a 100 g impure sample gives you only 95 g of usable CaCO3 for the reaction.
Do I use the impure mass or the pure mass in stoichiometry?
Always use the PURE mass in the balanced equation. The impurity does not take part in the reaction. Step 1: convert impure mass to pure mass using purity. Step 2: use only that pure mass with moles and the balanced equation. This is the single most common NEET mistake - students plug the whole impure mass in and get the wrong answer.
There are two directions - which formula do I use when?
There are two question types. (1) Given IMPURE sample, find product: pure mass = impure mass x (%/100), then find product. (2) Given the product or reactant needed, find how much IMPURE sample: first find pure mass needed, then impure mass = pure mass x (100 / %). One multiplies by %/100, the other divides. Read the question: are you starting from the dirty sample, or working backward to it?
How do I find % purity if I know pure mass and total mass?
Use % purity = (mass of pure substance / total mass of sample) x 100. For example, if 8 g of a 10 g sample is pure NaCl, purity = (8/10) x 100 = 80%. This matters for NEET because some questions give you the product formed and ask you to work backward to the purity of the starting sample.
Why does the impurity not react at all?
In NEET numerical problems the impurity is assumed to be chemically inert (it does not react with the other reactant) and non-volatile (it stays behind). So it just adds dead weight to the sample. This assumption lets you separate the pure part cleanly and treat only the pure mass in the equation.
⚠️ The NEET trap ✗ For 20 g of 20% pure limestone, students take the full 20 g as CaCO3: moles = 20/100 = 0.2, so CO2 = 0.2 x 44 = 8.8 g. ✓ Only 20% is pure CaCO3: pure mass = 20 x 20/100 = 4 g. Moles CaCO3 = 4/100 = 0.04. CO2 = 0.04 x 44 = 1.76 g. (NEET 2023 marked 2.64 g per the official key; the purity method itself gives 1.76 g - always apply purity BEFORE stoichiometry.) 🧠 Apply purity FIRST, stoichiometry SECOND. Never feed the impure mass straight into the equation.
Real NEET questions
NEET 2022
What mass of 95% pure CaCO3 is required to neutralise 50 mL of 0.5 M HCl according to the reaction: CaCO3(s) + 2HCl(aq) -> CaCl2(aq) + CO2(g) + 2H2O(l)?
A · 1.25 g
B · 1.32 g ✓
C · 3.65 g
D · 9.50 g
Solution: Moles of HCl = 0.5 x (50/1000) = 0.025 mol. From the equation, 2 mol HCl react with 1 mol CaCO3, so moles CaCO3 = 0.025 / 2 = 0.0125 mol. Mass of PURE CaCO3 = 0.0125 x 100 = 1.25 g. But the sample is only 95% pure, so we need a bigger impure mass: impure mass = pure mass x (100/95) = 1.25 x (100/95) = 1.32 g. Answer: (B) 1.32 g. Notice option A (1.25 g) is the trap - it forgets to correct for purity.
NEET 2023 Phase 1
The mass of CO2 produced by heating 20 g of 20% pure limestone is (at. mass Ca = 40): CaCO3 --1200 K--> CaO + CO2
A · 1.76 g
B · 2.64 g ✓
C · 1.32 g
D · 1.12 g
Solution: M(CaCO3) = 40 + 12 + 48 = 100 g/mol. Purity method: pure CaCO3 = 20 x (20/100) = 4 g = 0.04 mol, giving CO2 = 0.04 x 44 = 1.76 g (option A). The official NEET 2023 answer key, however, marked (B) 2.64 g, which corresponds to 0.06 mol CaCO3 (6 g pure). Learn the method (purity first, then stoichiometry) - the key value is reproduced here as officially released.
Solved Some Basic Concepts Of Chemistry NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Percentage purity = (mass of pure substance / total mass of impure sample) x 100. Rearranged, mass of pure substance = total mass x (% purity / 100).
Does impurity affect the number of moles of product?
Yes. Since only the pure part reacts, less pure substance means fewer moles of product. Always compute moles from the pure mass, never the impure mass.
How do I find the impure mass needed to make a certain product?
Work backward: first find the pure reactant mass needed from stoichiometry, then divide by the fraction pure. Impure mass = pure mass x (100 / % purity).
Is percent purity important for NEET?
Yes. Purity problems appear regularly in NEET (e.g. 2022 CaCO3 and 2023 limestone questions). They are easy marks if you remember to apply purity before doing mole calculations.
What is the difference between percent purity and percentage composition?
Percentage composition tells you how much of each element is inside a compound. Percent purity tells you how much of a whole sample is the wanted compound versus useless impurity. They are different ideas.
